Magnetic recording hard disk drives use a servo-mechanical control system to position the read/write heads to the desired data tracks and to maintain the heads on the tracks as required for read and write operations. Current hard disk drives use a rotary voice-coil-motor (VCM) as the actuator to position the read/write heads on the data tracks. Typically, each read/write head is attached to the end of a head carrier or air-bearing slider that rides on a cushion or bearing of air above the rotating disk. The slider is attached to a relatively flexible suspension that permits the slider to “pitch” and “roll” on the air bearing, with the suspension being attached to the end of the VCM actuator arm.
Special “servo” or head-positioning information is written in fields in circumferentially-spaced servo sectors in each of the concentric data tracks on each disk surface. The servo pattern is constructed across multiple tracks so that the read-back signal from the head, as it passes over the pattern, can be decoded to yield a head-position error signal (PES) that is used by the control system to maintain the head on the track during reading and writing of data.
The conventional servo pattern is a quadrature pattern that is decoded to yield two signals, a normal (P-PES) signal that is used primarily on one half of a track width and a quadrature (Q-PES) signal that is used primarily on the other half of a track width. At the quarter-track positions where the P-PES and Q-PES overlap, sometimes called the “stitching” regions, discontinuities between the P-PES and Q-PES signals can occur and the control system must select one or the other signal as the PES. The stitching gain, also referred to as the PES gain, is adjusted to ensure a continuous transition between P-PES and Q-PES at these quarter-track positions. Optimization of stitching gain results in greater accuracy of the PES reporting system which translates to improved track follow and seek performance.
The stitching gain is calibrated for each head at various tracks on the disk during a manufacturing process where a test computer is used to measure the P-PES and Q-PES at the quarter-track positions and then iteratively adjusts the PES gain to minimize the difference between the average P-PES and average Q-PES at those quarter-track positions. The resulting values of stitching gain, each associated with a corresponding head and track, are stored in memory in the disk drive and recalled during operation of the disk drive. However, for very high track densities, the asymmetry of the heads in the tracks and nonlinearities in the PES recording system can negatively influence the traditional averaging method, and result in less than optimal values of stitching gain. What is needed is a method that improves the robustness of the stitching gain calibration process.